Typical examples of such heating devices are: fusing devices provided in electrophotographic image forming apparatuses; drying devices for use in wet-type electrophotographic image forming apparatuses or ink jet printers; and erasing devices for rewritable media. A configuration of a fusing device provided in an image forming apparatus is described below as a representative example of conventional heating devices.
Generally, the fusing device has a heating roller and a pressure roller arranged on respective sides of a sheet transport path so that the rollers are pressed against each other. A fusing nip area is formed between the heating roller and the pressure roller as pressed against each other at a predetermined pressure. When passing through the fusing nip area, a sheet is heated and pressed, so that an unfixed toner image on the sheet is firmly fixed to the sheet. For the toner image to be properly fixed to the sheet, it is necessary to maintain the heating roller and the pressure roller at constant high temperatures. Accordingly, it is important that the heating roller and the pressure roller have heat-retaining properties.
FIG. 1 is a diagram illustrating a configuration of a conventional fusing device according to a fusing method using a heating roller. A heat reflector 238 is made of metal and has an arc-shaped cross-section. Provided for reflecting radiant heat from the heating roller 231, the heat reflector 238 is positioned so as to cover partially a circumference of the heating roller 231. The heat reflector 238 thus serves to reduce heat loss from the heating roller 231 and enhance heat-retaining properties of the roller 231, thereby allowing the fusing device to save energy (see Patent Literature 1).
However, conventional art including the Patent Literature 1 has not dealt with heat loss from heat reflectors. With reference to FIG. 1, focus is merely on reflecting radiant heat from the heating roller 231 by the heat reflector 238. Convective heat loss from an outer surface of the heat reflector 238, i.e., a surface thereof opposite to a surface facing the heating roller 231, has not been dealt with.
Therefore, a great amount of heat loss from the outer surface of the heat reflector 238 prevents the heating roller 231 from retaining heat. Especially for high-speed printing machines that allow 50 or more sheets to be printed per minute and a sheet to be transported through a fusing nip area in 23 or less msec, it is important that a heating roller and a pressure roller have heat-retaining properties. This is because fusing devices provided in such high-speed printing machines tend to encounter a problem of insufficiency of heat.
A feature of the invention is to provide a heating device that reduces convective heat loss from a heat reflector, thereby enhancing heat-retaining properties of a heating member. Patent Literature 1: JP H9-101700 A